Introduction
In Japan digital broadcasting has already been launched. BS (Broadcasting Satellite) started in 2000 and terrestrial digital broadcasting in 2003. All Japanese digital broadcasting is scrambled, but free to air, except for a few Pay TV channels. Content and copyright are protected by CAS. The function of CAS is implemented on a B-CAS card which is an IC card. The function of CAS is described later. Each STB has a particular B-CAS card. The B-CAS card is managed by BS Conditional Access Systems Co., Ltd. (cf. sources). Two types of B-CAS card exist: the red and the blue card. A red card is commonly used for BS, 110 degree CS, and terrestrial broadcasting. 110 degree CS is an independent pay TV service. A blue card is only for terrestrial broadcasting. If no B-CAS card is inserted in a STB, that STB cannot descramble scrambled content. The specification of these digital broadcasting depend on ARIB (Association of Radio Industries and Businesses standards; cf. sources).

The objectives of ARIB are to conduct investigation, research & development and consultation of utilization of radio waves from the view of developing radio industries, and to promote realization and popularization of new radio systems in the field of telecommunications and broadcasting. An important task of ARIB is the establishment of technical standards for radio systems in the field of telecommunications and broadcasting. Overall, ARIB aims at the promotion of public welfare.

The current state of digital broadcasting
10 million STBs were in use for BS digital in September 2005. When terrestrial digital broadcasting started in the Tokyo, Osaka and Nagoya areas on December 1, 2003, the number of terrestrial digital STBs was about 300.000. In the meanwhile more than 5 million terrestrial digital STBs are being used.

There are eight TV broadcasters including data broadcasting, four data broadcasters and five radio broadcasters in BS digital broadcasting. HDTV (high definition) and SDTV (standard definition) services are respectively seven and two channels.

The digital terrestrial TV broadcasts have also the high picture and sound quality of digital high definition (Hi-Vision) and attractive interactive features. Data broadcasting in Japanese characters provides information tailored to each locality. The digital terrestrial broadcasts are received by UHF antenna. The reception of sound and images is clear even on the STBs installed in moving trains, buses etc. A service for simple moving images, data and radio reception on mobile terminals etc. is also anticipated.

There are NHK and five commercial broadcasters which are major network TV companies and two local broadcasters in Tokyo area. Thus Japanese digital broadcasting which uses CAS is successfully spreading.

DRM in digital broadcasting systems
Japanese broadcasters encrypt content for copy protection, regional control of viewing, pay TV charging, etc. The encrypted content is transmitted to the subscriber's STB, which decrypts the encrypted content. Since each STB has a decryption key in its B-CAS card, it can decrypt content. It is possible to distribute different decryption keys to STBs in different areas, and thereby enable regional control of viewing. For pay TV, only the subscribers who sign a contract with a broadcaster can get a decryption key, and in this way broadcasters control access to the content.

The DRM standardized in Japan employs a three-step encryption system. The subscriber reveals his/her identity to a broadcaster and gets a B-CAS card. The B-CAS card is used as a tamper resistant module. Each B-CAS card has a unique master key, Km, that is stored in the tamper-resistant part of the card. Km is shared with broadcasters and is used to encrypt personal contract information when the broadcasters transmit information to a subscriber’s STB. Figure 1 shows a block diagram of the conventional DRM system for the Japanese digital broadcasting system.

In the broadcasting station, contents are scrambled with a scramble key, Ks. The scramble key is encrypted with a work key, Kw, and the work key is encrypted with a master key, Km. After that, the encrypted contents and keys are multiplexed and transmitted to the subscribers’ STBs. This procedure is called a three-step encryption.

The STB receives the encrypted contents and keys and de-multiplexes the encrypted content, scramble and work keys. It sends the encrypted session and work keys to the B-CAS card, which has been put in the STB. The B-CAS card decrypts the work key with the master key it holds, after which it decrypts the session key with the decrypted work key. The STB then gets the session key from the B-CAS card and decrypts the encrypted contents. In this way, subscribers can watch/listen to the content.

Of these three keys, Ks is changed every few seconds when the contents are encrypted to ensure security. Kw is the key that authorized subscribers get when they make a contract with a broadcaster. This key is updated with every contract. Km is a private key, and it is used to encrypt each contract when the contract information is sent to the B-CAS card. If broadcasters were to transmit Kw to all subscribers, they would need to encrypt and broadcast all the Kws. Such a broadcast would require a capacity in proportion to the number of subscribers, and thus it would impose a large load on the transmission channel. To decrease the load, Kw is broadcast only when it is to be updated. With these three keys and three-steps encryption, broadcasters can protect the copyrights of their contents. Moreover, to control the viewing region, as Kw is encrypted with Km and transmitted, broadcasters have to know each subscriber’s (B-CAS card’s) location.
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Figure 1: Conventional DRM system

Broadcasters then transmit the encrypted Kw to the subscribers that are in the region where the program is allowed to be viewed. This system can control viewing region. For pay TV, Kw is transmitted to subscribers who pay for programs or for channels. This system can realize pay TV.

Broadcasting System based on home servers
Broadcasting System based on home servers is a next-generation broadcasting system that utilizes a PDR (personal digital recorder) which is an STB with a large capacity storage, and it is now in the process of being standardized. It employs a four-step encryption. Figure 2 shows the block diagram of the proposed DRM for Broadcasting Systems based on Home Servers. It is assumed that the transmitted contents will be stored in the receiver, and it is required that conventional broadcasting services can be also received. Hence, the proposed DRM can be constructed by adding a new encryption key to the conventional DRM. The new key is called "content key" (Kc), and it is used to encrypt the session key when the content is stored in the PDR. Kc may be unique for each content. But actually Kc does not have to be unique for each content. It depends on the broadcaster.

Moreover, another new key is introduced. It is called "group key" (Km’). But Km’ will be called domain key with use home network. STBs with the same Km’ belong to the same domain. STBs in the same domain are able to use each other’s stored contents because they have the same encryption key Km’.

Km’ is set in the CAS card. As shown in Figure 2, Kc is encrypted with Km’ and stored in the STB. Ks is also encrypted by Kc. Since each STB’s Km’ is different from any other Km’s belonging to other CAS cards, once Kc is encrypted with a Km’ of a specific CAS card, it is impossible to decrypt the correct Ks by using a Km’ of another CAS card. Hence, it is impossible to decrypt correctly content from the stored encrypted content that is moved or copied from another subscriber’s STB.

Furthermore, Broadcasting System based on Home Servers uses Rights Management and Protection Information (RMPI). RMPI includes copy control information, playback control information, region information, output device control information, etc. When the content is used, the PDR checks the RMPI and controls its processes accordingly.
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Figure 2: DRM for Broadcasting System based on Home Servers


Content copy control
The realization of content protection and management in broadcasting requires a mechanism to execute some form of enforcement in a STB, which would operate according to content protection related control data transmitted along with regularly broadcast programmes. Digital broadcasting in Japan transmits encrypted content to achieve such enforcement, based on confidential data provided, including a decryption key. Such confidential information is provided in the form of an IC card (B-CAS card).

With regard to content protection and management, additional consideration should also be given to PDR that can record and reproduce digitally formatted programs without conversion (D-VHS, HDD, etc.). These recording and reproduction systems are designed on the premise of a high-speed digital interface (IEEE1394) connection, protecting digital content under a de facto standard (e.g. DTCP). For this reason, interfaces are also provided for transmitting content protection control data to recording devices and other systems over broadcast.

Regarding re-transmission to the Internet, a flag, or encryption mode, is prepared for a Content Availability Descriptor to enable receiver control.

The relationship between content protection and management requirements and a part of RMPI transmitted via broadcasting is described in the inserted Table 1. It prohibits a receiver from having the capability to send the designated contents, which either include a copy restriction by Digital Copy Control Descriptor’s "digital recording control data" or has copy protection specified by the Content Availability Descriptor’s encryption mode, to any output that could potentially allow the content to be re-transmitted over the Internet. Re-transmission to the Internet is prohibited in those cases where the encryption mode is "0" or copying is restricted by "digital recording control data.

Table 1: Copy control specification.
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Bottom line
Digital broadcasting receivers will be distributed with a key for broadcast viewing, on the condition that they operate according to the signals transmitted via the broadcast. Thus Japanese digital broadcasting is scrambled but free to air. This situation differs from the US and the EU as it is accomplished by CAS technique. CAS is mandated for ARIB (STD-B25) standard receivers. Scrambling contents does not necessarily require mandating a broadcast flag like in the US because the contents are protected by CAS.

Sources
  • Asia-Pacific Broadcasting Union (ABU): http://www.abu.org.my/public/index.cfm
  • Association of Radio Industries and Businesses standards: http://www.arib.or.jp/english/index.html. Relevant standards and guidelines developed are: – ARIB STD-B10 Service Information For Digital Broadcasting System – ARIB STD-B21 Receiver For Digital Broadcasting (Desirable Specifications) – ARIB STD-B25 Conditional Access System Specifications for Digital Broadcasting – ARIB TR-B14 Operational Guidelines for Digital Terrestrial Television Broadcasting – ARIB TR-B15 Operational Guidelines For Digital Satellite Broadcasting
  • BS Conditional Access Systems Co., Ltd.: http://www.b-cas.co.jp
  • The Association for Promotion of Digital Broadcasting: http://d-pa.org
  • The Association for Promotion of Satellite Broadcasting: http://www.bpa.or.jp

About the author: Kiyohiko Ishikawa received the Ph. D. degree in engineering from Nagoya University, Aichi, Japan in 2004. He joined Japan Broadcasting Corporation (NHK), Tokyo, Japan, in 1990, where he is now with the Science and Technical Research Laboratories. Since 1992, he has been engaged in research on magnetic recording head and media and signal processing for high-density magnetic recording systems and on broad-band and high-speed optical disk recording. He currently researches security system for digital broadcast based on home servers.

Status: first posted 25/01/06; licensed under Creative Commons
URL: http://www.indicare.org/tiki-read_article.php?articleId=166